Ok, so I recently was requested to write a detailed tutorial about skinning in GIMP. This is not meant to take away from any of the existing tutorials about skinning, just to supplement them. Be sure to check them out.
Just note that this is aimed at beginners, so a lot of the descriptions will seem really verbose. Anyway, here goes:

I'm going to talk about using a few little features to your advantage to make your new skin look as good as possible. First we will be skinning the blue Mandalorian Assault armor, called "N_Mandalorian01" in kotor tool to be precise. To extract it, just select it and press "Extract". Save it to your favorite directory and open it up in GIMP. Should look like this:

Now, the first thing people seek to do in skinning is obvious: change the color. Sometimes this can be a bit screwy. In order to basically "start from scratch" color-wise, go to the "Colors" tab and hit "Desaturate". You should get this now:

What I want to do now is get rid of those silver designs on the armor and give them some cool new ones. Getting rid of them is a bit of a pain. First, use the "Free Select" tool (the lasso) and trace around all of the spots you're going to want to get rid of. To select multiple areas, after you've selected one already, just hold shift as you select the rest of them. That way you will be adding them.

Now your areas should be selected like this:

What our goal here is to get the silver color we just selected to match the darker shade of grey of the rest of the armor so that it is solid. What you need to know here is that silver + black = grey. So, that being true, we are going to use the "Overlay" feature to fill in black overtop the silver to match the grey.

In some spots you may have to play with the opacity because it may come out too dark, but in this case 100% works well for most of the armor. I did change the opacity on some parts though, because they did in fact turn out dark.
So go ahead and fill in all of our selected spots to match the color of of rest of the armor and, after you finish, press ctrl+a to select the whole picture. It should look something like this:

You may notice that since we aren't perfect, there are a few spots where we selected that still have lighter or darker defining edges. There are a couple ways to fix this, the easiest and most basic of which is to use the "Smudge" tool to essentially blend the edges. Try to make it look as natural as possible, staying in one direction as if you were painting a door or something. Just go up or down, left or right, instead of going in circles or something. Also be sure to change the brush size for different parts until you get the effect you want.

Eventually you will get something looking more complete like this:

The next thing is to add your fancy new stripes or whatever you want to call them. As you may have guessed, you want to do exactly the opposite of what we just did. You want to select all the places you want stripes, and then, using the "Overlay" feature again, fill in your spots with white so that it matches the original stripe color. I came up with this:

Next, we obviously want to fill in the rest of the armor with a color. I want the color of the original red Mandalorian armor, so I am going to open up that file. Next, instead of trying to match the color manually, I can just ctrl+left click on the color and automatically select it. The next part is a pain, but once again use the "Free Select" tool and select all the parts of the armor you want to be that color. In my case, red. Then, using "Color" mode (located in the same place as overlay), fill it in. Now, "Color" mode only works with some colors. With certain colors, green in particular, it comes out very bright. If this is your case, just use the "overlay" mode again. Here's what I came up with:

(I also overlayed black at 60% opacity on the areas like the pants that you see darker)

So we are almost done, but what good is a brand new mando skin without a sweet logo? So go on google or something and find a logo you like. Now make sure it is saved as a .tga or .png so that it can have a transparent backgroud; erase the background, size it correctly, and then press ctrl+c to copy the image (make sure you are editing it in GIMP also). You will see the image appear in the brush selection box now.

Paste it onto your skin now:

One last thing to do, which is not at all necessary. I like to sometimes make my armor look battle-worn. You can use just about any of the brushes - play around with them to get the effect you want, but I will be using the one shown in the image, with the "scale" which is right under "opacity" set to .35, and with the opacity anywhere from 10-40, depending on the look you like.
Last, just to fit in with the battle-worn look, a blaster score or two would look pretty cool. To do this, just follow the same steps you followed to add your logo.

Then save the file and throw it into your override. Change your appearance to a Mandalorian to check it out!
(I changed the neck later because I thought it looked cooler )

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The next little thing I want to go over is how to get a decent Camouflage look. For the sake of the tutorial I'm going to use a template I just made to show you guys. If you want, you can follow the first part of the tutorial to get to here (or leave the parts you want camo desaturated):
Now find or make a camo texture you like. Or you can use mine:

Select all the parts you want camo, once again using the "Free Select Tool".
Here's the important bit. Select the "Clone" tool.

Now Ctrl+Click on your camo texture. It is now "selected" to clone. Then, with the clone tool still on, select "Overlay" like we did before. Now just fill in your selected areas on your armor skin with the camo texture:

Now you may have noticed that you might have ran out of room on your camo texture and had to re-click it, resulting in the unnatural edges as shown by the arrows in the picture above. To get rid of them just use the "smudge" tool we used earlier. For this, a circular motion helps. Make sure you haven't clicked out of your selection of your camo still!
You may also notice that while you can see all the cloth folds and such (which is the point of overlay), it may look "dull". We're about to fix it up.

After you fixed your edges, go to the "colors" tab and click "Brightness/contrast".
Now, play with both of these slider bars until you get a look you like. This is a pretty dark skin to begin with, so it didn't take much tweaking. However, with other armors, it may take a lot more and adjusting the contrast will greatly enhance the look of your camo. How you should approach it is to adjust the contrast first, and then adjust the brightness as needed. My configuration:
(I also upped the contrast seperately on the helmet because it was brighter than the rest)

Now throw it into your override and see how it looks!

I really hope that this tutorial helps some people in their quest for an awesome skin. I hoped to point out a few key features that help me all the time. If you have any questions feel free to contact me